This invention relates to a reinforcing fiber material used in fiber reinforced plastics (hereinafter referred to as FRP). More particularly, this invention relates to a laminated reinforcing fiber material suitable for use in relatively large FRP.
As regards fiber substrates such as, for example, woven fabric or its prepreg, which are used as reinforcing fiber materials for FRP, not so thick substrates can be obtained. Accordingly, they are usually used in the form of a laminate consisting of a plurality of fiber substrates.
However, when such a laminate is heated under pressure to form FRP, since there is nothing to restrain the fiber substrates from moving relative to each other, the fiber substrates or the reinforcing fibers are pushed aside by a resin flow, and the arrangement of the reinforcing fibers are disturbed. This disturbance in arrangement tends toward increasing, particularly, in case where unidirectional prepregs in which the bond between the reinforcing fibers is performed by resin only is employed or in case where a resin injection molding process consisting of resin intrusion is adopted for a material having a relatively strong texture, such as a woven fabric.
On the other hand, when a quasiisotropic FRP is desired, fiber substrates are laminated so that, for example, directions of their reinforcing fibers cross at an angle of 0,.+-.45.degree.or 90.degree. . When this is done, since the reinforcing fibers and the resin have markedly different coefficients of linear thermal expansion, residual stresses due to the difference in thermal strain arise between layers. Furthermore, since the Poisson's ratio of FRP is dependent upon the direction of the arrangement of reinforcing fibers and has a great anisotropy within the surface, the difference in Poisson's ratio between layers, when cross-laminated, becomes considerably large. Accordingly, when a stress is given to FRP, stresses due to the above-mentioned thermal strain difference or Poisson's strain difference, in addition to an external force, are exerted in a complicated manner, delamination by resin rupture between layers occurs before the reinforcing fibers are broken. Especially, when high elongation carbon fibers (fibers having an elongation of about 1.7-2.2%) are used as reinforcing fibers, the above-mentioned disadvantages become marked, because there is a great thermal strain difference due to a marked difference between their coefficients of linear thermal expansion (the coefficient of linear thermal expansion of carbon fiber is -(0.7-1.2).times.10.sup.-6 /.degree.C. and that of resin is about (55-100).times.10.sup.-6 /.degree.C.), and because there is a greater difference in Poisson's strain due to its high elongation. Furthermore, the more fiber substrates a laminate has, that is, a thicker FRP has more resin layers between fiber substrates, the more marked the above-mentioned problem becomes, causing a decrease in reliability of FRP. Moreover, once a crack arises between layers, it propagates at a stretch because there is nothing to prevent the propagation.